There are two major types of absorption refrigeration equipment in commercial use: (1) air cooled systems using ammonia as the refrigerant and water as the absorbent, and (2) water cooled systems using water as the refrigerant and lithium bromide as the absorbent.
Although these are the major types in commercial use, and there are many patents relating to these and other types, variations have been patented from these general principles and the following are typical examples of such patents: U.S. Pat. Nos. 4,055,964--Swenson et al. 2,350,115--Katzow.
Others have demonstrated air cooled absorption refrigeration systems using other absorbent, refrigerant pairs. The following patents relate to these systems: U.S. Pat. Nos. 4,433,554--Rojay et al. and 3,483,710--Bearint.
Still others have patented water cooled refrigeration systems using other salts or other salts in combination with lithium bromide as the absorbents. The following are examples of these: U.S. Pat. Nos. 3,609,086--Modahl et al. and 3,541,013--Macriss et al.
Water cooled refrigeration circuits using the double effect generator are also in commercial use and have been patented as seen in the following patents: U.S. Pat. Nos. 3,495,420--Loweth et al., 3,389,573--Papapanu et al., 4,183,228--Saito et al., and 2,182,453--Sellew.
In absorption refrigeration and/or heating systems, the generator, sometimes called desorber, is a very important part of the system and contributes significantly to the overall efficiency. Much attention has been given to the construction of these devices, and various arrangements are shown in the following patents: U.S. Pat. Nos. 3,323,323--Phillips, 3,608,331--Leonard, and 4,127,993--Phillips, and 4,424,688--Wilkinson.
These existing air cooled absorption refrigeration circuits have demonstrated cooling coefficients of performance as high as 0.50 using various absorbent/refrigerant pairs. These systems have also been demonstrated as heating only heat pumps with a coefficient of performance of up to 1.3.
As used herein, coefficient of performance; i.e. COP, is defined as the energy transferred at the load in BTU/unit of time over the energy provided to the system in BTU/unit of time which is well understood by those skilled in the art.
Air cooled refrigeration circuits have also been demonstrated which can be reversed to provide either heating or cooling to an air conditioned space (a load) by switching the flow of an intermediate heat transfer solution typically consisting of water and antifreeze solutions such as ethylene glycol, etc.
Liquid cooled absorption refrigeration circuits using the double effect generator cycle to achieve high efficiency are commercially available. However, these systems are not suitable for use in heating a conditioned space (the heating load) since the refrigerant freezes at 32.degree. F. and therefore cannot be used in a space heating system at ambient temperatures below approximately 40.degree. F.
Absorption refrigeration and heat pump systems are well known in their basic operating characteristics and need little further description except to establish the definitions and context in which this invention will be later described.
In a typical system a refrigerant, water or other phase change material is dissolved in a absorbent (typically lithium bromide or other salts) and these are often called the "solution pair". The refrigerant is absorbed or desorbed (expelled) in or out of solution with the absorbent to varying degrees throughout the system and the heat of absorption is added or extracted to produce heating and cooling effects.
The solution pair enters a generator where it is subjected to heat and the applied heat desorbs (expells) the refrigerant water in the form of a vapor which is conveyed to the condenser. There, external ambient cooling condenses the refrigerant vapor to liquid, which is conveyed through an expansion valve, into an evaporator where heat is gained. In the refrigeration system operation the heat gained in the evaporator is from the cooling load.
The low pressure vapor then passes to an absorber where ambient cooling allows the absorbent solution to absorb the refrigerant vapor. The solution is then conveyed to a recuperator by a pump. The recuperator is a counterflow heat exchanger where heat from the absorbent/refrigerant solution, flowing from the generator to the absorber, heats the returning solution pair flowing from the absorber to the generator. In the heating cycle, the cooling applied at the absorber and/or the condenser is the heat delivery to the heating load.
As a matter of convenience and terminology herein, each part of the absorption system which operates at the same pressure is termed a chamber.
Conventional absorption refrigeration/heating systems are two chamber systems although three chamber systems appear in the prior art and have seen limited use. When operated as a heat pump two chamber systems give respectable heating performance but give poor cooling performance.
Using ammonia (NH.sub.3) as the refrigerant and water (H.sub.2 O) as the sorbent, heat pumping can occur from an ambient air source which is at temperatures below freezing. In a theoretical assessment where the air is treated as if it were dry so that no defrosting is necessary, the typical two chamber NH.sub.3 /H.sub.2 O heat pump would represent a significant improvement over what would be expected of a simple furnace. However, since heat pumps are more expensive than furnaces, cooling season performance benefits are needed to justify the added expense. In other words, the heat pump must act as an air conditioner also to offset the cost of a separate installation of an air conditioner with the furnace.
For cooling, an NH.sub.3 /H.sub.2 O system is predicted to have a COP equal to about 0.5. This low performance index causes unreasonable fuel (or energy) costs from excessive fuel (or energy) use. This low performance of the ammonia/water system results from the poor performance characteristics of the ammonia/water solution at the higher temperature ranges, if the heat is supplied to the absorption system at higher temperatures.
Three-chamber systems of various types have been suggested which would improve the performance by staging the desorption process into effects. This would allow for increasing the actual temperature at which the driving heat is added to the system (cycle). The reference Carnot cycle efficiency would be increased and the real cycle would follow suit. Until the present invention it was thought that this increase in temperature would represent an unreasonably high pressure, especially for ammonia/water systems, and would force the system to operate in regions for which data is not readily available.
In addition the pressure has tended to rule out ammonia/water in a three-chamber system. The search for organic material such as halogenated hydrocarbons and other refrigerants as a replacement for the ammonia has been limited by fluid stability at these higher temperatures. Normal organic refrigerant stability tests have indicated that it is necessary for oil to be present for operation in vapor compression refrigeration systems. These high operating temperatures rule out most of the common refrigerants, particularly being heated directly by combustion products which often cause local hot spots, which result in working fluid degradation and/or corrosion of components.
U.S. Pat. No. 4,441,332--Wilkinson is an example of a four-chamber absorption refrigeration system to provide refrigeration and/or heat pump total capability. This prior art patent employs two chemically separated two-chamber systems which are mechanically integrated into a total system to take advantage of the high performance of one solution pair in a low temperature range for cooling and the advantages of the other solution pair in a high temperature range when the total system is heat pumping in the heating mode.
The invention described herein is an integrated three-chamber system having one solution pair using an organic material of unusual fluid stability at higher temperatures when manipulated in an apparatus and system to take advantage of its properties. The typical preferred solution pair for operation as part of the system and components of this invention is ammonia as the refrigerant and sodium thiocyanate as the absorbent.
Others have given consideration to this solution pair as examplified by the ASME publication "Performance of A Solar Refrigeration System Using Ammonia--Sodium Thiocyanate", by Swartmen et al., in Nov. 1972 and the publication entitled "A Combined Solar Heating/Cooling System", by Swartmen and presented July 28-Aug. 1, 1975 at the 1975 International Solar Energy Congress and Exposition and U.S. Pat. No. 3,458,445--Macriss et al.
The heat actuated, air cooled, double effect generator cycle absorption refrigeration system of this invention overcomes limitations of the existing prior art technology. The air cooled system of this invention eliminates the need for cooling water and the use of ammonia as the refrigerant avoids refrigerant freezing during heating operation. The double effect generator cycle permits high efficiency through internal heat recovery in the absorption refrigeration circuit. The use of sodium thiocyanate as the absorbent eliminates the need for analyzers and rectifiers to purify the refrigerant stream. Internal refrigerant flow reversal, to achieve heat/cool switching and defrosting, eliminates the need for intermediate water/antifreeze heat transfer loops to switch from heating to cooling operation.